This invention relates generally to internal combustion engines and, more particularly, to a throttle control assembly for use in an internal combustion engine having spark advance speed control.
In the engine art, it is common to vary the speed of a 2-cycle internal combustion engine through direct variation of engine ignition timing and indirect variation of throttle valve position. Typically, a rotatable cam is moved in accordance with desired engine speed changes and a cam follower, engaging the cam, is coupled to the throttle valve to adjust throttle valve position in accordance with the degree of cam rotation. However, due to practical manufacturing tolerances as well as shrinkage problems when the cam and cam follower are manufactured of economical plastic, the maintenance of a wide-open throttle valve position at high engine speeds has been difficult to achieve with consistency. Over-center operation of the throttle valve, i.e. operation of the throttle valve so that it rotates to and then beyond the wide-open position, sometimes results. This, in turn, causes engine deceleration and reduced overall engine performance.
One technique for avoiding over-center operation of a throttle valve includes splitting the throttle cam and providing an adjustment screw on the cam itself to permit adjustment of the cam profile, i.e., the radial length between the camming surface of the cam and the center of rotation of the cam. Another known technique includes providing the cam follower with a similar adjustment mechanism so as to permit adjustment of the cam follower length. Sometimes it is necessary to combine both techniques in order to provide cam follower contact at the proper cam pick-up point and to assure proper correlation of throttle valve position with engine ignition timing. However, these prior techniques do not assure that over-center operation of the throttle valve will not occur.
Attention is directed to the following U.S. Patents:
Griffin et al.; U.S. Pat. No. 4,476,068 Oct. 9, 1984
Boeger; U.S. Pat. No. 4,462,268 July 31, 1984
Teramura et al.; U.S. Pat. No. 4,443,388 Apr. 17, 1984